1. Field of the Invention
This invention relates to an improved thermoplastic polyurethane and to a process for the preparation thereof. More particularly, this invention is concerned with a thermoplastic polyurethane suitable for producing artificial leathers having improved properties such as moderate elasticity, resistance to microorganisms, cold, stickiness and nitrogen oxide gas, and with a process for producing the same.
2. Description of the Prior Art
Heretofore, artificial leathers consisting mainly of a non-woven, woven or knitted fabric and an elastic polymer have been made as a substitute for natural leather. The mechanical and physical properties, as well as the practical durability, of these artificial leathers depend upon not only the kind(s) of fibers comprising the fabric, the structure of the non-woven fabric and the physical properties of the woven or knitted fabric, but also depend greatly upon the properties of the elastic polymer.
In order to make an artificial leather having various desirable properties, therefore, it is necessary to use an appropriate elastic polymer.
As one such elastic polymer, polyurethanes have been used which consist of a soft segment of a long-chain diol such as a polyester diol or a polyether diol, and a hard segment derived from a low-molecular-weight diol such as ethylene glycol, propylene glycol or 1,4-butanediol, or a diamine such as ethylene diamine or 1,2-propylene diamine, and an organic diisocyanate.
Such polyurethanes have excellent flexibility, bending durability, abrasion resistance, toughness and resistance to chemicals, and hence are widely used for artificial leathers. These polyurethanes, however, have disadvantages:
(1) Since these polyurethanes have too high elasticity, an artificial leather prepared using these polyurethanes has an elasticity similar to rubber, and does not exhibit a natural leather touch. PA1 (2) Since the resistance to microorganisms of these polyurethanes is poor, the durability of an article obtained using these polyurethanes is not good enough to provide an article suitable for practical use. For example, the surface of shoes prepared therefrom will crack when worn for a long time. PA1 (3) Since these polyurethanes have high stickiness and a high frictional coefficient, the touch or feel of an artificial leather obtained therefrom is not good when they are used as a surface finishing polymer for an artificial leather. PA1 (4) These polyurethanes have poor resistance to cold and tend to be yellowed by a gas such as nitrogen oxide gas.
As described above, these known polyurethanes which consist of a soft segment of a long-chain diol and a hard segment derived from a low-molecular-weight diol or a diamine and an organic diisocyanate are not necessarily suitable for artificial leathers. Methods have been proposed to avoid the disadvantages of these known polyurethanes, which methods comprise adding a third component to such known polyurethanes to improve the basic properties of these polyurethanes.
Japanese Patent Application Laid-Open No. 51533/75, Morita et al, published May 8, 1975, discloses a polyurethane composition having improved stickiness which contains a polar, organic fluorine compound.
Japanese Patent Application Publication No. 22053/75, Shirota et al, published July 28, 1975, discloses a polyurethane composition having improved stickiness which contains isophthalic acid or terephthalic acid components in the polyurethane molecule, which can be used for preparing an artificial leather having a good feel. However, these polyurethane compositions do not have other good properties such as moderate elasticity or resistance to microorganisms, cold, and nitrogen oxide gas which are necessary in polyurethanes used to prepare excellent artificial leathers.
U.S. Pat. No. 3,681,291, Khan, issued August 1, 1972, discloses a liquid castable urethane composition consisting of the reaction product of: (I) a prepolymer obtained by reacting a diisocyanate with a polyol such as polyoxypropylene polyol with (II) a curing composition comprising (a) a polyol such as polyoxypropylene polyol, (b) an aromatic diol such as an adduct of bisphenol A and propylene oxide, (c) an aromatic amine corresponding to the formula ##STR5## wherein n represents a value between 0.1 and 0.3 and (d) an organometallic catalyst. Since this urethane composition contains an aromatic amine having more than two active hydrogens as a curing agent, the polyurethane obtained has a very high molecular weight and high strength, and is insoluble in organic solvents. Although this polyurethane is suitable for motor mounts, vibration dampers, oil seals, gaskets, fuel hose and machine pads, it is not useful for artificial leathers.